<svg xmlns="http://www.w3.org/2000/svg" 
     xmlns:xlink="http://www.w3.org/1999/xlink"
     onload="boom();">
     
<script type="text/javascript">

function boom() { 
  var greensquare = document.getElementById("greensquare");
  var use = document.getElementById("use");
  greensquare.appendChild(use);
}

</script>

<rect id="greensquare" x="10" y="10" width="10" height="10" fill="green" />

<use xlink:href="#greensquare" id="use" />

</svg>
